Not sure where the term "Good" came from in relation to this day - some say it came from "God's" Friday. But undoubtedly it was a good day for all of humanity.
It was also finally a "good" day for Jesus Christ, for it was the culmination of His whole work on earth - the very reason He became man.
For on that day, Christ died so that we might have eternal life, in obedience to His Father's will for Him.
On the morning of Good Friday he was scourged at the Pillar, crowned with thorns, mocked and beaten, made to carry His Cross to Calvary; as weak as He was from the beatings and loss of blood, he was hindered in the carrying of the Cross by being pulled this way and that, and many times falling to the ground, with the Cross crashing down on his back and head.
Then in the last horrible act of barbarism, He was nailed to the Cross, through his hands and feet, and swung aloft, where He suffered unbelievable agony for three hours before He died.
He did all this to open the gates of Heaven to all believers. Knowing this, how can we possibly sin against Him?